Low loss SAW double-mode structure suppressing spurious radiation [EGSM filter applications]

The transverse-double mode interdigital transducer (IDTs) structure on 42° Y-X LiTaO₃ was selected for low loss EGSM filter testing. We first observed a leaky-SAW radiation pattern. SAW radiation patterns were obtained using a heterodyne interferometer-type laser probe method. Two spurious radiation modes were observed in the conventional double-mode-structure. The first mode wave traveled in almost the same direction its the main SAW response on the bus-bar edge. The direction of the second mode wave was rotated 40° with respect to the main SAW direction. We succeeded in suppressing the first spurious mode by using a grating bus-bar structure which reduces the SAW velocity on the bus-bar edge. The first spurious wave was not observed and low loss frequency characteristics were measured in the new IDT structure. The grating bus-bar aperture was optimized by testing the shape factor and the insertion loss. The new structures improved the performance of the filter.